1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charging device uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to foam an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a pair of looped belts or rollers, one being heated by a heater for melting toner (hereinafter referred to as “fixing member”) and the other being pressed against the fixing member (hereinafter referred to as “pressing member”). In a fixing process, the fixing member and the pressing member meet and press against each other, forming a so-called a fixing nip through which a recording medium is passed to fix a toner image thereon under heat and pressure.
Toner used in such fixing devices generally contains resin material. When melted in the nip portion, the toner in the toner image on the recording medium tends to stick to the fixing member, winding around the fixing member even after the recording medium exits the nip portion, causing a paper jam. To address such difficulty, a wax component is added to the toner, or alternatively, the fixing member is covered with a release agent such as silicon oil, to prevent the toner in the toner image sticking to the fixing member.
In recent years, however, because of difficulty in handling of the release agent such as the silicon oil, application of such a release agent on the fixing member becomes less frequent, complicating efforts to separate reliably the recording medium bearing the toner image from the fixing member.
To counteract such a difficulty, to facilitate separation of the recording medium bearing the melted toner from the fixing member, a separation mechanism including a separation claw is proposed to separate physically the recording medium undesirably wound around the fixing member from the fixing member. Disadvantageously, such a separation claw slidably contacts the fixing member while rotating, leaving a trace of slide on the surface of the fixing member, thereby yielding a resulting image with streaks. Furthermore, because the position of the separation claw is fixed, the distance between the tip of the separation claw and an end of the nip portion changes as the pressing member moves to accommodate different types and thicknesses of the recording media sheets, thus hindering reliable separation of the recording medium from the fixing member.
In view of the above, a contactless separation plate disposed very close to the fixing member is proposed. In order to obtain a similar reliable separation ability as the separation claw that directly contacts the fixing member, the space between the contactless separation plate and the fixing member needs to be minute and adjusted by 0.1 mm, for example. Furthermore, the separation plate needs to be disposed as close to the nip exit as possible.
In a related-art belt-type fixing device, such a contactless separation plate is disposed close to the nip exit downstream in a direction of conveyance of the recording medium. The separation plate does not move in accordance with changes in the position of the fixing belt. Since the separation plate does not move in conjunction with the movement of the fixing member, the distance between the nip exit and the tip of the separation plate varies as the fixing member moves. That is, the relative positions of the separation plate and the fixing member change as the fixing member moves, complicating efforts to adjust accurately the slight gap between the tip of the separation plate and the fixing member. Similarly, as the pressing member moves in accordance with the types and thickness of the recording medium, the gap between the nip exit and the tip of the separation plate also varies, thereby also hindering separation of the recording medium.
Another example of separation of the recording medium after exiting the nip portion includes a peeling pad disposed inside an inner loop formed by the belt-type fixing member after the end of the nip, to change a curvature of the looped fixing member after the nip exit. In this configuration, a change in the curvature of the fixing member after the nip portion bends a conveyance path of the recording medium, thereby facilitating separation of the recording medium as it exits the nip portion. However, similar to the foregoing configuration, the peeling pad does not move in conjunction with the pressing member that moves in accordance with types and thickness of the recording medium. Therefore, the relative positions of the peeling pad and the pressing member change, thereby changing undesirably the gap between the peeling pad and the nip exit, and thus hindering reliable separation of the recording medium from the fixing member.
Even if both the peeling pad and the contact-less separation member such as the separation plate are employed in a fixing device, the relative positions of the peeling pad and the separation member relative to the nip exit change when the position of the pressing member moves.